This invention relates to photosensitive elements and particularly to photographic, photothermographic, and polymeric elements made with multilayer polymeric supports.
Light-sensitive recording materials, such as photographic and photothermographic elements, frequently suffer from a phenomenon known as halation which causes degradation in the quality of the recorded image. These light sensitive recording materials typically comprise a photosensitive layer and a support (also known as a substrate or film base). Image quality degradation occurs when a fraction of the imaging light which strikes the photosensitive layer is not absorbed, but instead passes through to the support on which the photosensitive layer is coated, and a portion of the light reaching the support is reflected back to strike the photosensitive layer in a different area. This reflected light may, in some cases, contribute significantly to the total exposure of the photosensitive layer. However, such increased exposure (speed) can occur at the expense of sharpness.
Any particulate matter, including silver halide grains, in the photosensitive element may cause light passing through the element to be scattered. Scattered light which is reflected from the support will, on its second passage through the photosensitive layer, cause exposure over an area adjacent to the point of intended exposure. This effect leads to reduced image sharpness and image degradation. Silver-halide containing photosensitive materials, including photographic and photothermographic elements, are particularly prone to this form of image degradation since the photosensitive layers contain light-scattering particles (see, T. N. James, The Theory of the Photographic Process, 4th Edition, Chapter 20, Macmillan 1997).
In order to improve the image sharpness of photographic and photothermographic elements, it is customary to incorporate into one or more layers of the material a dye which absorbs light that has been scattered within the coating and would otherwise lead to reduced image sharpness. To be effective, the absorption of this dye must be at about the same wavelength as the sensitivity of the photosensitive layer.
In the case of imaging materials coated on a transparent support, a light absorbing layer is frequently coated in a separate backing layer or underlayer on the reverse side of the support from the photosensitive layer. Such a coating, known as an xe2x80x9cantihalation layer,xe2x80x9d effectively reduces reflection of light which has passed through the photosensitive layer. A similar effect may be achieved by interposing a light absorbing layer between the photosensitive layer and the support. This construction, known in the art as an xe2x80x9cantihalation underlayerxe2x80x9d is applicable to photosensitive coatings on non-transparent as well as on transparent supports.
A light absorbing substance may be incorporated into the photosensitive layer itself to absorb scattered light. Substances used for this purpose are known as xe2x80x9cacutance dyes.xe2x80x9d These dyes should not cause fogging of the silver in the imaging layer. It is also possible to improve image quality by coating a light absorbing layer above the photosensitive layer of a photographic element. Coatings of this kind, described in U.S. Pat. Nos. 4,581,323 and 4,312,941 prevent multiple reflections of scattered light between the internal surfaces of a photographic element.
Use of a reflective support is known to be one method of increasing film speed. However, reflective supports may not reduce halation, are generally opaque, and do not allow for viewing the image using back lighting.
U.S. Pat. No. 5,795,708 describes the use of a dichroic mirror antihalation layer to increase speed and improve sharpness for heat processable films. The continuous dichroic mirror layer is formed from multilayers of alternating layers of silicon dioxide and titanium dioxide and is coated on top of a base layer (i.e. a support).
Multilayer polymeric stacks have also been disclosed that function as wavelength selective reflectors such as xe2x80x9ccold mirrorsxe2x80x9d that reflect visible light but transmit infrared or xe2x80x9chot mirrorsxe2x80x9d that transmit visible and reflect infrared. Examples of a wide variety of multilayer stacks that may be constructed are included in U.S. Pat. No. 5,882,774 entitled xe2x80x9cOptical Film.xe2x80x9d U.S. Pat. No. 5,882,774 is incorporated herein by reference.
An additional problem occurs when a high quality photothermographic or photographic medium is exposed by coherent radiation (e.g. as in a laser imaging system) at a uniform exposure level across the entire surface of the sheet. This problem, termed xe2x80x9cwoodgrainxe2x80x9d is a spurious pattern that bears a remarkable resemblance to the pattern of grain in wood, hence the name. These patterns tend to be neither symmetrical nor repetitive, and, like the grain in polished wood, appear as variations in the optical density (i.e. lightness and darkness) on the image surface. Such spurious image patterns are, of course, undesirable in any imaging system where the quality of the image is critical.
Typically, one source of woodgrain is an interference pattern set up within the photosensitive layer by light reflecting from its upper and lower surfaces. The lower surface of said layer may be weakly reflecting but it can reflect enough light to cause problems in some cases. For example, with a polyester support, the index of refraction is about 1.65 while a silver halide photographic emulsion coating has an index of about 1.5. This index differential is enough to cause some woodgrain. If a reflector is placed at this bottom interface, much more light is reflected and this may enhance the woodgrain effect.
The interference pattern that is created by light reflected from the upper and lower surfaces of the photosensitive layer is a map of the thickness of said layer. Polymer films and coatings generally can be controlled in thickness to within a few percent. One percent of a thick film is much greater than one percent of a thin film. Therefore, for typical film or coating variations, when the interfering waves originate from the surfaces of thick films, the interference fringes occur much closer together than for surfaces of thin films. The thicker the films, the closer the interference fringes until they eventually merge, i.e. they dissappear. Interfering rays from the two surfaces can be either constructive or destructive, depending on the separation distance of the two surfaces. The standing wave pattern induced by this situation causes a higher absorption of light in areas where constructive interference occurs. Since the difference between the constructive or destructive interference condition is only xc2xd wave, or about 200 nm for 633 nm light within material of index 1.5, any small variation in thickness can change the interference condition from constructive to destructive, that is, from enhanced light intensity to decreased light intensity. The thickness of a typical coating may vary by several xc2xd waves over a short lateral distance, giving rise to several high and low intensity exposure bands, or fringes, over said distance. Polymer films and coatings generally can be controlled in thickness to within a few percent. One percent of a thick film is much greater than one percent of a thin film. Therefore, for typical film or coating variations, when the interfering waves originate from the surfaces of thick films, the interference fringes occur much closer together than for surfaces of thin films. The thicker the films, the closer the interference fringes until they eventually merge, i.e. they dissappear.
The present invention is a photosensitive element comprising a layer of a photosensitive material on a transparent, multilayer, polymeric support. The photosensitive material is sensitive to actinic radiation of certain wavelengths. The multilayer polymeric support comprises numerous alternating layers of at least two different polymeric materials. The multilayer polymeric support preferably reflects at least 50% of actinic radiation in the range of wavelengths to which the photosensitive material is sensitive. This photosensitive structure has increased speed over standard photosensitive structures coated on polymeric supports.
Preferably, the photosensitive element is a photographic element or a photothermographic element. The multilayer polymeric support is preferably transparent when viewed by the human eye. The multilayer polymeric support may have a relatively thick intermediate film layer and numerous thin alternating layers of two or more different polymeric materials. If the numerous, thin, alternating layers are located at the surface of the polymeric support closest to the photosensitive layer, the support not only provides increased speed but also provides an antihalation effect.
In another aspect of the present invention, a multilayer reflector is located a distance removed from the photosensitive layer sufficient to reduce any woodgrain that would otherwise occur, while increasing speed. The multilayer film may be placed on the bottom side of the support. Typically, the support is rather thick, on the order of 0.005 inches or more (0.125 mm or more), so the multilayer reflector may alternatively be placed in the center of the support. Alternatively, the multilayer reflector may itself serve as the support. The reflector itself could be 0.01 mm, or thinner, depending on the indices of refraction of the materials comprising the reflector.